Phase Inverter blew? (C+)

[Platypus]

So I decided to replace the 5881's in my C+ with some 6L6GE's to see if I could get some shimmer back into the amp...

I replaced the two inner sockets and when I fired it up it made an awful popping sound followed by a large WHOOSHing ocean type sound. I immediately put it on standby and let it sit for a second and tried it again.. this time it worked and the tone was KILLER. Then I tried the clean channel, anything above 3 on my volume caused that awful distorted whoosh and deep crackle (like a raging fire).

So I put it on standby to test and see if it was a fluke and it kept doing it over and over. I decided to check the preamp tubes because that was my first thought. Bingo. The 12AX7 closest to the front and furthest left if you look behind the amp (next to the 105) was making an awful racket when I tapped it with my pencil eraser tip. Like a static popping sound..

I yank V5 out and there is a bent pin on it :roll: :roll: :roll:

Luckily I had a spare NOS bugle boy that came with this amp so for grins I decided to pop that baby into V5 to see if I can get it back to life. It works perfectly now without the popping on standby and everything (I hope) is A-OK with it. As an added bonus, my reverb control came back to life (see my other thread, it didnt come on till 8 on the knob before)??

Two questions....

1) Is it purely coincidence that replacing those power tubes caused my phase inverter to go south on me? The bent pin is a rather large smoking gun but it worked for weeks in this condition before today when I relaced the tubes. Is it possible that the tubes sent it over the edge to it's death because they pull more current?

2) The 12AX7 in question was an old school Mesa labled ECC83/12AX7. It literally just says MESA in cap letters with no logo whatsoever so I'm guessing it's an original from 84 when this amp came out. For some reason this tube had more bite to it than the NOS one I stuck in there to test with. It had a bit more crunch if that makes sense. I want to get a balanced 12AX7 for this slot since I doubt this ambiguous NOS tube I have is balanced or even proper for this slot.

Any other suggestions/comments would be helpful. I almost had a heart attack :shock:

 

[JOEY B.]

The 12AX7 closest to the front and furthest left if you look behind the amp (next to the 105)

That is V4, and guess what functions it has? :shock:

 

[Platypus]

The 12AX7 closest to the front and furthest left if you look behind the amp (next to the 105)

That is V4, and guess what functions it has? :shock:

This diagram is very misleading then..
http://mesaboogie.com/manuals/Mark%20II-C%20Manual%20-%20Maintenance%20&%20Repair.pdf

My first thought was the reverb since the reverb was SUCKING before. It's not perfect now but it's a lot better than it was.

Just to make sure we're talking about the same tube here.

There are 4 in the back all in a row.. then there's one that's much further out, it's RIGHT next to the 105 tranny.

If it is indeed V4 it would make much more sense because the lead drive was going from awesome to washed out and crappy and the clean mode was distorting way too early and in a VERY unpleasant way.

 

[Platypus]

Furthermore, can you tell me the exact V position of all the tubes? Starting at the back right near the input jack going left? I'm definitely not saying you're wrong but reread my positioning because I can see how you'd think I was talking about V4 here.. but it's the one right in front of that fourth tube so I think it's V5??

The previous owner put 2 SPAX7's in the first two lots if the first two tubes are on the far right and if those aren't the lead stages then I'm even more confused. It skips a tube and V4 is furthest out? :idea: :shock:

 

[JOEY B.]

I must have misunderstood your post. I thought that you meant furthest to the left and closest to the front of the amp (looking from the rear). My mistake.

 

[Platypus]

I must have misunderstood your post. I thought that you meant furthest to the left and closest to the front of the amp. My mistake.

No problem, so it is indeed V5 then joey?

If so, did I shock it by putting in these 6L6GE's?

Did I do any damage by running them, should I take them out? (hope not, the tone is godlike)

Would the PI mess with the reverb and/or the gain of the amp?

 

[Platypus]

Gotcha, ok sorry joey based on my nomenclature you were correct.

However now that we're sure it's V5, do you guys know the answers to my two questions?

 

[JOEY B.]

I almost had a heart attack :shock:

I've had at least 6 Boogie induced "heart attacks" in the last few years. The best one was when I saw smoke coming from the EQ sliders of my '85 C+. :lol: I can laugh now, but there was no humor that night.

As far as V5 affecting gain, I have heard some people talk of using the 12AT7 in V5 to lower gain. The reverb situation has me stumped :?: .

 

[Platypus]

Haha, smoke would be a bit worrisome indeed!


What I'm most curious of is, did adding these new power tubes blow my V5 out or was it coincidence? Am I safe to run the amp as is?

 

[JOEY B.]

I ran #6 GT-6L6-GE's in the amp now known as "Old Smokey" without any issues, even in Class-A with the volume cranked. I think that bad luck and bad timing may have met at your doorstep.

 

[phyrexia]

I bought some new tubes to go to a mini-gear-fest a friend was holding, and the PI tube lasted maybe 15 minutes. it was probably just 'your time'

 

[CoG]

I'm one of those people who uses a 12AT7 in V5 even though I am generally playing pretty high-gain stuff, my MkIII has enough places to get gain from that I didn't want another that I don't have a knob for.

This dude:

http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf

says to change your PI almost as often as your power tubes. I haven't got the technical background to say if he's right, I am just throwin' it out there.

 

[Restless Rocks]

So let me understand this: You replaced a tube with A BENT PIN and that solved the problem. Why would you suspect the power tubes to be responsible for this? You may have other tubes that are improperly seated in their sockets.

 

[Platypus]

So let me understand this: You replaced a tube with A BENT PIN and that solved the problem. Why would you suspect the power tubes to be responsible for this? You may have other tubes that are improperly seated in their sockets.

We're not talking about a drastic bend here and I had never pulled this tube before yesterday. It is an original mesa tube from the 80s I imagine. I am simply suspect of the power tubes because I'd been playing with this amp for about a week with zero problems and the moment I replaced them this happened. The preamp tube still went into the socket just fine but it had a crook to one of the pins.

I'm not one to believe in sheer coincidence very often so I was just curious if perhaps those particular power tubes would have pushed this V5 over the edge in any way. (electronically, pulling more power, etc?) I'm not too savvy on the nitty gritty so I just figured I would ask. All the other tubes are fine (I checked them all after this).

 

[Platypus]

The PI article (linked in CoG's post) was very interesting and perhaps indicates a relationship between Power Tubes and the PI that would cause the PI to blow.

Which part in particular?

 

[Boogiebabies]

This is interesting.

In this amp, everything depends on the V2. It's the effects loop, reverb signal mixer, lead output, Master Volume, GEQ signal out and is the signal out to the phase inverter. The only tube left to alter the signal to the power amp is the PI. If one of the two triodes in the PI was going south, it would effect the preamp signal to the power amp. The PI in the C+ is run pretty hard and can go at any time without warning or cause. Like any other preamp tube, it would exhibit the same symptoms of lack of output, sputtering, noise and distortion. If you changed the V5 and the amp came back, it's totally possible because the whole preamp is dependent on it to pass the signal to the power amp. I would not relate a power tube change to the PI going down. With six different sets of power tube brands tested in three amps, the one constant was the 483 volts to the plate. The 105 is very consistent. A bent or dirty pin could cause a dead spot so I would straighten the pin and spray it with contact cleaner. You can try it in another position to rule it out.

 

[Platypus]

This is interesting.

In this amp, everything depends on the V2. It's the effects loop, reverb signal mixer, lead output, Master Volume, GEQ signal out and is the signal out to the phase inverter. The only tube left to alter the signal to the power amp is the PI. If one of the two triodes in the PI was going south, it would effect the preamp signal to the power amp. The PI in the C+ is run pretty hard and can go at any time without warning or cause. Like any other preamp tube, it would exhibit the same symptoms of lack of output, sputtering, noise and distortion. If you changed the V5 and the amp came back, it's totally possible because the whole preamp is dependent on it to pass the signal to the power amp. I would not relate a power tube change to the PI going down. With six different sets of power tube brands tested in three amps, the one constant was the 483 volts to the plate. The 105 is very consistent. A bent or dirty pin could cause a dead spot so I would straighten the pin and spray it with contact cleaner. You can try it in another position to rule it out.

Ed, thanks for the info!!

It was definitely a bad triode because it would work sometimes and sometimes not during my testing. I noticed that moving my guitar cable would cause it to fluctuate in and out of this weirdness. That NOS tube we talked about a few weeks ago is in its place and everything is back to normal 100% now.

1) Do you have any recommendations for this slot? I did some research and called some folks and most landed on the Sovtek LPS 12ax7 as being the best choice aside from NOS.

2) The reverb.. seems quite weak on this amp comared to my 60/100C+. I have to put it on about 7 or 8 for it to really be noticeable. Is there something that may have gone wrong here in addition the PI? I can't remember testing the reverb before I replaced the other components so it's hard to say if it was like this before hand but if it were tube related I imagine the gain would suck on the lead channel since V3 and 4 do reverb and 1/2 of the lead drive. Is it possible for one of the pairs in one of these tubes to go bad, IE the reverb part, but still have the gain be normal or would it just stop working altogether? It works.. but it's very subtle.

Thanks again.

 

[Boogiebabies]

I almost had a heart attack :shock:

I've had at least 6 Boogie induced "heart attacks" in the last few years. The best one was when I saw smoke coming from the EQ sliders of my '85 C+. :lol: I can laugh now, but there was no humor that night.

As far as V5 affecting gain, I have heard some people talk of using the 12AT7 in V5 to lower gain. The reverb situation has me stumped :?: .

Each position has an effect on the gain and tone. I'll go through about 20 preamp tubes before I find my spot.
Of course, when we have an issue with the lead channel or reverb we think 99% of the time the V3 or V4. We also have to deal with the unknowns of gain and transconductance along with NOS factor. Either new or NOS there will be the good, bad and ugly.

 

[Platypus]

I almost had a heart attack :shock:

I've had at least 6 Boogie induced "heart attacks" in the last few years. The best one was when I saw smoke coming from the EQ sliders of my '85 C+. :lol: I can laugh now, but there was no humor that night.

As far as V5 affecting gain, I have heard some people talk of using the 12AT7 in V5 to lower gain. The reverb situation has me stumped :?: .

Each position has an effect on the gain and tone. I'll go through about 20 preamp tubes before I find my spot.
Of course, when we have an issue with the lead channel or reverb we think 99% of the time the V3 or V4. We also have to deal with the unknowns of gain and transconductance along with NOS factor. Either new or NOS there will be the good, bad and ugly.

Interesting, it seems the NOS tube is much less bright and has a very different gain structure. It's amazing how much V5 effects the actual tone.. I always thought it was rather utilitarian but man, it was very different with those two tubes. I tend to like the brighter mesa one in there but the gain is so 3D with this NOS tube.

 

[Boogiebabies]

This is interesting.

In this amp, everything depends on the V2. It's the effects loop, reverb signal mixer, lead output, Master Volume, GEQ signal out and is the signal out to the phase inverter. The only tube left to alter the signal to the power amp is the PI. If one of the two triodes in the PI was going south, it would effect the preamp signal to the power amp. The PI in the C+ is run pretty hard and can go at any time without warning or cause. Like any other preamp tube, it would exhibit the same symptoms of lack of output, sputtering, noise and distortion. If you changed the V5 and the amp came back, it's totally possible because the whole preamp is dependent on it to pass the signal to the power amp. I would not relate a power tube change to the PI going down. With six different sets of power tube brands tested in three amps, the one constant was the 483 volts to the plate. The 105 is very consistent. A bent or dirty pin could cause a dead spot so I would straighten the pin and spray it with contact cleaner. You can try it in another position to rule it out.

Ed, thanks for the info!!

It was definitely a bad triode because it would work sometimes and sometimes not during my testing. I noticed that moving my guitar cable would cause it to fluctuate in and out of this weirdness. That NOS tube we talked about a few weeks ago is in its place and everything is back to normal 100% now.

1) Do you have any recommendations for this slot? I did some research and called some folks and most landed on the Sovtek LPS 12ax7 as being the best choice aside from NOS.

2) The reverb.. seems quite weak on this amp comared to my 60/100C+. I have to put it on about 7 or 8 for it to really be noticeable. Is there something that may have gone wrong here in addition the PI? I can't remember testing the reverb before I replaced the other components so it's hard to say if it was like this before hand but if it were tube related I imagine the gain would suck on the lead channel since V3 and 4 do reverb and 1/2 of the lead drive. Is it possible for one of the pairs in one of these tubes to go bad, IE the reverb part, but still have the gain be normal or would it just stop working altogether? It works.. but it's very subtle.

Thanks again.

Triodes can be all over the place. One side could be at 100% and the other could be at 70%. Unless you buy the preamp tubes tested and gain rated the only way to test is by ear using many tubes. The V1 and V5 in all my MK's is the GT 12AX7M. It's been very reliable and consistent and to me, warms the amp up a bit. The rest are Ruby Chinese 12AX7C9's.

The reverb issues do not have to be related to the preamp.
On the first C+ I ever bought the reverb was barely audible.
I cleaned everything, did the one wire mod, checked the circuit values and cleaned the cables. I sprayed the jacks, reverb tank jacks and it was all for nothing. I tried the reverb resistor mod and still nothing. I did not have another MK series yet, so I could not swap the tanks. I called my guy at Mesa and he asked it was a combo with the reverb in the bag. It was, and he proceeded to tell me that the glue used to attach the foam in the tanks would evaporate and lay on the springs. I opened the tank up and sure enough, the springs were seriously sticky and had bits of foam stuck to them. I cleaned the springs with an orange based cleaner and got all the bits of foam out and amazingly the reverb was back.
On another C+, all it needed was the one wire ground mod and it was huge !!! I would start with the mod and move on to either swapping tanks or cleaning the one you have. the static may be from your reverb circuit trying to find a clean path to ground. It can also cause a nasty fuzzy distortion and noise. This too can sound like a preamp tube going bad.